1. Field of the Invention
The present invention relates to a verification of an electronic scale, more particularly a method for calibrating a large fixed electronic scale which belongs to a metrology verification technique.
2. Description of the Related Art
Fixed electronic scales are metrical weighing instruments with the most mature technique around the world and are in a mass production as well as a broad application. They are commonly applied in varies loading vehicles and goods measurement adapted to metallurgy, chemical industry, railway, port and industry and mining enterprises. They are also adapted to a trading settlement and a process control of the weighing process during the manufacture. Therefore, the fixed electronic scales are ideal metric or measurement instruments for modern enterprises to increase the weighing system. The principle of the fixed electronic scales is operated by setting the weighed objects or loading vehicles on a scale table. The scale table transmits the gravity to a swinging bearing such as a steel ball or a press head under the gravity force, and a spring member of the weighing cell becomes deformed, which makes a strain gauge bridge lose the balance and outputs an electric signal in a positive proportion to a weight value. The signal is then amplified via a linear amplifier, then converted into a digital signal via an A/D, and thence processed by a microprocessor of a gauge to display a weight number directly.
The fixed electronic scale needs to go about the verification before the use in order to check its accuracy class. Further, the large scale also has to verify again to check the accuracy level after the scale is used for a period of time or equipped with replaced elements so that the scale can be properly adjusted to meet the requirement of the accuracy. A standard instrument applied in calibrating conventional fixed electronic scales is mainly divided into three kinds. The national standard no.GB7723-2008 of the fixed electronic scale, adopting the international criteria of OIML R76 “non-automatic scale” (2006E), explicitly points out three permissible standard equipments, namely (1) a weight, more specifically standard weights or standard quality; (2) an auxiliary verification device, more specifically a scale equipped with an auxiliary verification device or an independent auxiliary device; and (3) a substitute for standard weights in verification, more specifically part standard weights and other random fixed loads replacing standard weights.
However, in the regulation of verification of JIG539-1997 titled by “numeral indicator scale”, the standard equipments as stipulated are divided into two types: (1) a standard weight; and (2) a standard weight and a substitute of the standard weight. Therefore, the common used standard equipments for calibrating and verifying fixed electronic scales are the standard weight or the standard weight as well as its substitute, and the auxiliary verification device is not adopted as the standard equipment to calibrate the fixed scale. The international criteria of R76 titled by “non-automatic scale” and the national standard adapted to the fixed electronic scale no. GB7723-2008 provides the auxiliary verification device with a simple stipulation that if the scale is equipped with any auxiliary verification device or an independent auxiliary device is used for the verification, the maximum permissible error of the device should be one-third of the maximum permissible error of the verified load. There is no exact definition for the auxiliary verification device cited in the aforementioned international criteria and GB7723-2008 standard except for the above stipulation stating a maximum permissible error of the auxiliary verification device. Until now, it is still rarely to find out documentations related to the application of auxiliary verification device in calibrating large fixed electronic scales around the world.
A disclosure as published by China patent no. CN86105843 on 1988, Feb. 17 and titled by “a verification device for truck scale and track scale”. This disclosure mainly discloses a verification device without using a weight, but the accuracy of the quasi-pressure gauge as disclosed fails to comply with the accuracy requirement as stipulated. Another disclosure as published by China utility innovation no. CN02230837.7 on 2003, Jan. 22 and titled by “large scale gauge”. This disclosure also discloses a scale verification gauge without using a weight. More specifically, this disclosure includes a verification cell (4), a display gauge (7), a pressure device, and a pressurizing support (3). The pressurizing support is integral with a base of a verified scale. The pressure device is fixed onto the pressurizing support. The metric accuracy of the verification cell and the display gauge is larger than the metric accuracy of the verified scale. The verification cell is disposed on a scale body (9) of the verified scale. The verification cell and the pressure device are connected by a sphere and an output of the verification cell is connected to the display gauge. The pressure of the pressure device is applied to the cell and displayed by the display gauge. The pressure is concurrently applied to scale body of the verified scale and displayed by a scale gauge. The two displayed value are thence compared to detect the calibration error of the verified scale. However, the verification device as disclosed can only detect the in-service cells applied in the scale one by one. The verification device is effectively a superposition force standard machine. The pressure device and the pressurizing support load manually, which cannot satisfy the requirement of a load fluctuation (force source stability) and a force stability retentive time stipulated in “JJG734-2001 regulation of verification of force standard machines” and “JJG144-2007 regulation of verification of standard gauges”. The verification span focuses on verifying the loading value of every in-service cell of the scale, not on verifying the span of the scale. The verification accuracy of the scale is related to the accuracy of every cell and is also related to a rigidity of the scale table, a foundation of the scale table, an accuracy of the gauge, and a junction box. Although the in-service cell of the scale is eligible, the metric property of the scale may not be eligible. Therefore, it is not satisfied by only working on the full verification of the metric property of the scale. The factors affecting the scale accuracy such as the deflection of the scale table, the foundation of the scale table, the accuracy of the gauge, and the junction box should be additionally considered during the verification. The disclosure can only detect the cell, so the verification process cannot explain by analogy with the effective weighing state and can only go about the analogous comparison of similarity between the in-service cells of the scale. The most important issue is that the disclosure cannot verify the scale directly.
The current method for calibrating a fixed electronic scale is described. First, take a verification of a fixed electronic truck scale weighed 100 tons as an example and apply the standard weight and the substitute of the standard weight to proceed the verification according to the fixed electronic scale national standard no. GB7723-2008 or to the requirement of regulation of verification stipulated by JJG539-1997, “digital indicator scale”. Wherein, referring to FIG. 1, it is noted that a fixed electronic truck scale 2′ in a specification of 100 tons and with conditions of three sections with 18 meters in length, e=50 kg, m=2000 is defined. The disclosure includes a scale display gauge 21′, three scale table boards 211′,212′,213′, eight cells 231′-238′, eight cell support points 241′-248′ disposed above respective cells, and eccentric load testing areas 251′-258′ correspondingly disposed around respective cell support points as shown by dotted line in FIG. 1. While verifying, put the standard weight or the substitute of the standard weight 3′ on each eccentric load testing area to execute an eccentric loading test sequentially. The specific process for the verification of the metric property includes steps of:
1. Pre-pressurizing: pre-applying the load to 100 t at once or using a loading vehicle not less than 50 t to go back and forth a loader not less than 3 times;
2. Accuracy of the zero setting and tare device;
3. Zero setting before loading;
4. Weighing the property:
4.1 Applying the standard weight and the substitute during the verification to check the quantity of the standard weight and execute the repeatability test of the scale. First, check the repeatability of the weighing point at 50 t and apply the standard weight weighing 50 t to the loader three times. If the error of the repeatability is not larger than 0.3 e, the standard weight 3′ can be reduced to 35% of the maximum weighing measure. If the error of the repeatability is not larger than 0.2 e, the standard weight 3′ can be reduced to 20% of the maximum weighing measure;
4.2 Weighing test: add the weight or the substitute 3′ from a zero to 100 t in a sequence from the smallness to the bigness and remove the weight to return to the zero by the same way. At least five verification points, such as it, 25 t, 50 t, 75 t, and 100 t should be at least chosen for testing;
4.3 Tare weighing test: at least two different tare weights are detected by a tare weighing test. According to step 4.2, the five test points are it, 50 t, the changed scale measure of the maximum permissible error, the possible maximum net weight, and 80 t;
4.4 Eccentric loading test: put the standard weight 3′ weighing 14 t on the eight eccentric load testing areas 251′-258′ by turns for testing until errors of indicating values of the eight eccentric load testing areas 251′-258′ are all not larger than 50 kg;
4.5 Discrimination test: execute the test at the weighing points 1 t, 50 t and 100 t while executing the verification; and
4.6 Repeatability test: prepare and test two respective groups at 50 t point and a point close to the maximum point (90 t). Each group is repeatedly tested at least three times.
From the above verification, it needs to transport a tonnage corresponding to the weight of the standard weight or the substitute. For example, (1) the weight of 100 t is carried in the pre-pressurizing step; (2) the weight of 150 t is carried in the 4.1 step for executing the repeatability test of the scale while adopting the standard weight and the substitute to check the quantity of the standard weight; (3) the weight of 100 t is carried in the 4.2 step for the weighing test; (4) the weight of 160 t is carried in the 4.3 step for the tare weighing test; (5) the weight of 112 t is carried in the 4.4 step for the eccentric loading test; and (6) the weight of 270 t is carried in the 4.6 step for the repeatability test.
Therefore, disadvantages attendant on the conventional method applying the standard weight or the standard weight and the substitute to calibrate the fixed electronic scale are described:
1. Heavy workload of the verification and low efficiency. To detect an eligible 100 t fixed electronic truck scale would require moving the standard weight and the substitute which are total 932 t in weight. If it is not eligible, an adjustment is needed. After the adjustment, the re-verification is executed and the weight is moved again. The process of moving the weight or the substitute weighing over thousand tons is inevitably required.
2. Poor safety of moving a large number of weights or substitutes. Due to the limited loading table of the electronic truck scale, such as 54 square meters in area for verifying a truck scale weighing 100 tons, it is difficult to put the weight or the substitute weighing 100 tons on the limited area and is also dangerous for loading and unloading the weight or the substitute.
3. Hard to seek the suitable substitute. Not all users of large electronic truck scales can find suitable substitutes. For example, the standard scale installed aside the highway is hard to find the proper substitute, and users at the railway, the port, containing liquid poison, gaseous chemical industry, textile factory, coal mine, etc. are also hard to find suitable substitutes.
4. Hard to transport the standard weight. To detect a 100 t truck scale would require transporting the standard weight weighing at least 50 t. To detect a 150 t truck scale would require transporting the standard weight weighing at least 75 t. In the practical operation nowadays, it can only transport the weight weighing 15 t at once. The limited amount of the transportation at once is especially carried out in the montane district including a bridge limit load, a road limit load, a geography limit load, an installation in the gully (such as at the mine), etc.
5. High costs. To transport and move such a large number of standard weights or substitutes would require many verification scale vehicles and hoists and spend days and labors fulfilling the verification. For example, to detect a 100 t truck scale generally requires 7 working days.
To sum up, most of county agencies, city agencies and provincial agencies are devoid of sufficient standard weights for calibrating the large scale such as the 150 t electronic truck scale. Even if the quantity of the standard weight is sufficient, the safety for loading, unloading and transporting the weight and the transporting cost can not be assured under the present technique. Further, although the weight is successfully transported to the target place, to go about the verification in light of the regulation of verification of JIG539-1997 “digital indicator scale” still requires a heavy workload and takes a large amount of time to detect, which renders the verification unable to be assured of meeting the regulation. Therefore, the conventional method for calibrating a large fixed electronic scale by applying the standard weights or the standard weight and the substitute still requires an improvement.